1. Field of the Invention
The present invention relates to an apparatus and method for controlling knock in an internal combustion engine.
2. Description of the Related Art
When the ignition timing of the internal combustion engine is too advanced, knocking occurs due to abnormal combustion, which can damage the cylinders and the pistons. Some internal combustion engines are thus provided with a knock control apparatus to prevent knocking. The knock control apparatus determines whether knocking has occurred, and regulates operations of the internal combustion engine to prevent knocking.
The determination by the knock control apparatus depends on a signal output from a knock sensor. A typical knock sensor is a vibration pickup that is mounted onto the cylinder block. The knock sensor senses vibrations of the cylinder block in the frequency range specific to knocking and converts the vibrations into an electrical output signal. The knock control apparatus determines that knocking occurs when a knock intensity value LVPK, which is converted from the output signal of the knock sensor, is equal to or greater than a knock criterion level VKD. In contrast, the knock control apparatus determines that knocking does not occur when the knock intensity value LVPK is lower than the knock criterion level VKD. In accordance with the determination result, the knock control apparatus controls the internal combustion engine to prevent knocking, in other words, the knock control apparatus executes a knock prevention control. The knock prevention control is generally achieved by a responsive ignition timing control that results in more effective knock prevention. More specifically, the ignition timing is retarded when knocking is detected or advanced when knocking is not detected. This allows the knock control apparatus to control the internal combustion engine such that knocking is minimized within the allowable range or the engine conditions are maintained at the level immediately before knocking occurs.
The cylinder block vibrates not only due to knocking, but also due to driving the injectors, valves, and other components of the internal combustion engine. Thus, the output signals of the knock sensor include a true knock signal generated due to knocking as well as a false knock signal generated due to mechanical operations of the engine components. Therefore, if the false knock signal is improperly determined as the true knock signal, the ignition timing is unnecessarily retarded, which can lower the engine power output and the emission efficiency.
In view of this problem, the following knock control apparatuses for an internal combustion engine have been proposed. Japanese Patent Application Publication No. JP-A-Hei 6-101556 describes a knock control apparatus that calculates the angle of ignition timing retardation, performed under the retardation control, based on the output signal of the knock sensor, and determines whether the output signal of the knock sensor is a false knock signal based on the calculated retardation angle. To be more specific, the knock control apparatus presets a certain maximum threshold and minimum threshold. If the retardation angle falls within the range between the preset maximum threshold and the preset minimum threshold, the output signal is determined to be a false knock signal. In contrast, if the retardation angle falls out of the range, the output signal is determined to be a true knock signal.
Another knock control apparatus for an internal combustion engine is described in JP-A-2003-278592. The knock control apparatus described in JP-A-2003-278592 determines whether a signal, output from the knock sensor when knocking occurs is a true knock signal based on the knock intensity LVPK. The knock intensity LVPK is a value converted from the output signal of the knock sensor. More specifically, if the current knock intensity value LVPK is equal to or lower than its previous value, the output signal is determined to be a true knock signal. In contrast, if the current knock intensity value LVPK exceeds its previous value, the output signal is determined to be a false knock signal. In addition, if true knocking is detected, retarding the ignition timing prevents the knocking. This results in lower knock intensity LVPK. In contrast, if false knocking is detected mainly due to mechanical noise from the pistons and other components, retarding the ignition timing helps the false knocking grow louder. This results in higher knock intensity LVPK. Thus, the knock control apparatus determines if true or false knocking is detected based on the knock intensity LVPK, obtained under the ignition timing retardation control, or on the behavior of the output signal of the knock sensor.
However, a number of mechanical devices are mounted in the internal combustion engine so that mechanical noises also cause the cylinder block to vibrate. Vibrations of the cylinder block vary with the engine operating conditions. Due to this, the knock control apparatus, described in JP-A-Hei 6-101556, cannot avoid the problem of improper determination, such that the false knocking is improperly determined as true knocking, if the angle of ignition timing retardation, which is calculated based on the output signal of the knock sensor, falls out of the range between the preset maximum threshold and the preset minimum threshold. In turn, the knock control apparatus, described in JP-A-2003-278592, also has the similar problem, such that the output signal that indicates false knocking is improperly determined as true knocking, if the current knock intensity LVPK is equal to or lower than its previous value. As described above, any type of knock control apparatuses may not ensure sufficient accuracy of knock determination when the knock determination is based on the output signal of the knock sensor.